Guru Gobind Singh Ji explained:
Reminder to do Right things- Kara is usually is worn on the dominant hand, it comes to notice when doing most of our work in daily life. For example- an officer going to sign an important document, the kada comes to his notice while doing so, reminding him that his signature should not lead to anyone's suffering.
Another important function of kada was to serve as a weapon during battles. Multiple kadas were worn in single arm to attack others as well as defend from sword attacks from others.
But nowadays Kada has been reduced to only a cultural thing without any meaning just like Turban.
Equality- It is mandatory to wear kada made of pure iron only. Wearing gold or silver or any other kadas made from expensive metals is not permitted, hence social equality. Also, both men and women wear it, hence, gender equality.
Scientifically, keeping iron and steel on our body strengthens our electromagnetic field.